Ultrafast THz photoconductivity of photovoltaic polymer-fullerene blends: a comparative study correlated with photovoltaic device performance
Edwin J. Heilweil, Jin Zuanming, Dominik Gehrig, Clare Dyer-Smith, Frederic Laquai, Mischa Bonn, Dmitry Turchinovich
Ultrafast photo-induced carrier dynamics in prototypical low band-gap polymer:fullerene photovoltaic blend films PTB7:PC70BM and P3HT:PC70BM is investigated using ultrafast terahertz (THz) spectroscopy. The sub-picosecond (ps) and few-ps decays of THz-probed photoconductivities for both compounds are observed, attributed to the rapid formation of polaron pairs by exciton-exciton annihilation and subsequent polaron pair annihilation, respectively. The transient THz photoconductivity spectra of PTB7:PC70BM are well described by the Drude-Smith (DS) model, directly yielding the important charge transport parameters such as charge carrier density, momentum scattering time, and effective localization. By comparison with P3HT:PC70BM, we find that in PTB7:PC70BM the mobile charge carrier photoconductivity is significantly enhanced by a factor of 1.8 and prevails for longer times after charge formation, due to both improved mobile charge carrier yield and lower charge localization. In PTB7:PC70BM a strong dependency of electron momentum scattering time on electron density was found, well parameterized by empirical Caughey-Thomas model. The difference in ultrafast photoconductivities of both P3HT:PC70BM and PTB7:PC70BM is found to correlate very well with the performance of photovoltaic devices based on those materials.